LED Resistor Calculator — Find the Right Resistor for Your LED Circuit
Every LED requires a current-limiting resistor in series to protect it from excessive current. Use the formula R = (Vs − Vf) / If — subtract the LED's forward voltage from your supply voltage, then divide by the desired current. This calculator does that instantly and also shows the nearest E24 standard resistor value, power dissipation, and actual current.
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Frequently Asked Questions
Why does an LED need a resistor?
An LED (Light Emitting Diode) is not a resistive device — it has an exponential current-voltage relationship. Without a series resistor, a small increase in voltage causes a huge surge in current that instantly destroys the LED. The resistor limits current to a safe operating level, typically 10–30 mA for standard indicator LEDs.
How do I calculate the resistor value for an LED?
Use the formula R = (Vs − Vf) / If, where Vs is the supply voltage, Vf is the LED forward voltage, and If is the desired current in amps. For example, a 5 V supply with a red LED (Vf = 2 V) at 20 mA: R = (5 − 2) / 0.02 = 150 Ω. Then pick the nearest E24 standard resistor value.
What is the forward voltage of an LED?
Forward voltage (Vf) is the voltage drop across an LED when it is conducting. It depends on the LED color: red LEDs are typically 1.8–2.2 V, green 2.0–2.4 V, blue and white 3.0–3.4 V, and yellow 2.0–2.2 V. Always check the LED's datasheet for the exact value at your operating current.
What current should I use for an LED?
Standard 5 mm and 3 mm indicator LEDs are typically rated at 20 mA maximum. For longer life and less heat, run them at 10–15 mA. High-brightness and power LEDs may handle hundreds of milliamps — check the datasheet. The calculator defaults to 20 mA, a safe choice for most general-purpose LEDs.
How do I wire multiple LEDs in series?
When LEDs are wired in series, all forward voltages add up. The formula becomes R = (Vs − (Vf × n)) / If, where n is the number of LEDs. For example, three 2 V LEDs on a 12 V supply at 20 mA: R = (12 − 6) / 0.02 = 300 Ω. The supply voltage must exceed the total forward voltage for any current to flow.
What wattage resistor do I need for an LED?
Calculate the power dissipated by the resistor with P = (Vs − Vf) × If. For a 5 V supply, 2 V LED at 20 mA: P = 3 × 0.02 = 60 mW. A standard ¼ W (0.25 W) resistor is more than adequate. For safety, choose a resistor rated at least twice the calculated power — so use ¼ W for up to 125 mW of dissipation.
What is an E24 standard resistor value?
The E24 series is a standardized set of 24 resistor values per decade (e.g., 100–910 Ω, 1k–9.1k Ω). Manufacturers produce resistors at these specific values, so calculated resistance values are rounded to the nearest E24 value when purchasing. Common E24 values include: 100, 110, 120, 130, 150, 180, 200, 220, 270, 330, 390, 470, 560, 680, 820, 910 Ω and so on.
Can I connect LEDs in parallel with one resistor?
No — never connect multiple LEDs in parallel with a single shared resistor. Small differences in forward voltage between LEDs cause unequal current sharing; the LED with the lowest Vf will take most of the current and burn out. Each parallel branch must have its own series resistor to independently limit current in each LED.
LED Resistor Formula
Every LED needs a current-limiting resistor to prevent excessive current from destroying it. The required resistance is calculated by dividing the voltage available across the resistor by the desired LED current:
Basic Formula (single LED)
R = (Vs − Vf) / If
Example: Vs = 5 V, Vf = 2 V, If = 20 mA → R = (5 − 2) / 0.02 = 150 Ω
Series LEDs Formula
R = (Vs − (Vf × n)) / If
Where nis the number of LEDs. Example: 12 V, 3 red LEDs (2 V each), 20 mA → R = (12 − 6) / 0.02 = 300 Ω
Power Dissipation
P = (Vs − total Vf) × If
The power dissipated by the resistor — used to select an appropriate wattage rating.
- Vs — supply voltage (V)
- Vf — LED forward voltage (V) per LED
- If — desired forward current (A) — typically 10–30 mA for standard LEDs
- n — number of LEDs in series
- R — resistance in ohms (Ω)
How to Choose an LED Resistor
- Find your LED's forward voltage (Vf) — check the datasheet. Typical values: red 1.8–2.2 V, green 2.0–2.4 V, blue/white 3.0–3.4 V, yellow 2.0–2.2 V.
- Decide the desired current (If) — most 5 mm and 3 mm indicator LEDs run at 10–30 mA. High-brightness LEDs may allow up to 350 mA. Check the datasheet maximum rating and stay below it.
- Calculate R— use the formula R = (Vs − Vf) / If and find the nearest E24 standard value.
- Check power rating— calculate P = (Vs − Vf) × If. A standard ¼ W (0.25 W) resistor is sufficient for most low-current LED circuits. Use ½ W or 1 W resistors if P exceeds 0.1 W.
- Always round up— when selecting the nearest E24 value, prefer the next higher value over the next lower one to avoid exceeding the LED's current rating.
Common LED Forward Voltages
| LED Color | Typical Vf (V) | Typical If (mA) | R for 5 V supply (Ω) |
|---|---|---|---|
| Red | 2.0 | 20 | 150 |
| Yellow | 2.1 | 20 | 145 |
| Green | 2.2 | 20 | 140 |
| Blue | 3.2 | 20 | 90 |
| White | 3.2 | 20 | 90 |
| Infrared (IR) | 1.2 | 20 | 190 |
Note: Forward voltage varies by manufacturer and LED model. Always check the datasheet for the exact Vf at your operating current.
Series vs Parallel LED Wiring
Series Wiring (Recommended)
LEDs wired in series share the same current path, so only one resistor is needed. The total forward voltage is the sum of all individual Vf values. Series wiring is preferred because all LEDs receive the same current, producing uniform brightness.
R = (Vs − (Vf1 + Vf2 + ... + Vfn)) / If
Requires: Vs > total Vf (otherwise no current flows)
Parallel Wiring (Avoid if Possible)
Parallel LEDs each need their own current-limiting resistor because small differences in Vf between LEDs cause unequal current sharing. Never connect LEDs in parallel with a single shared resistor — one LED will take most of the current and burn out.
Power Rating Guide
The resistor must be rated to handle the power it dissipates as heat. Use the formula P = (Vs − total Vf) × If to calculate power, then choose a resistor with a wattage rating at least twice the calculated value for reliable long-term operation.
| Power Dissipated | Recommended Resistor Rating |
|---|---|
| < 62 mW | ⅛ W (0.125 W) |
| 62–125 mW | ¼ W (0.25 W) — most common |
| 125–250 mW | ½ W (0.5 W) |
| > 250 mW | 1 W or higher |
E24 Standard Resistor Values
The E24 series contains 24 values per decade (e.g., 10–91 Ω, 100–910 Ω, 1k–9.1k Ω). This calculator automatically finds the nearest E24 value to the exact calculated resistance. The E24 base values are:
1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1
× any power of 10 (0.1, 1, 10, 100, 1k, 10k, 100k, 1M Ω...)
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